title: Probing carrier lifetimes in photovoltaic materials using subsurface two-photon microscopy.
abstract: Accurately measuring the bulk minority carrier lifetime is one of the greatest challenges in evaluating photoactive materials used in photovoltaic cells. One-photon time-resolved photoluminescence decay measurements are commonly used to measure lifetimes of direct bandgap materials. However, because the incident photons have energies higher than the bandgap of the semiconductor, most carriers are generated close to the surface, where surface defects cause inaccurate lifetime measurements. Here we show that two-photon absorption permits sub-surface optical excitation, which allows us to decouple surface and bulk recombination processes even in unpassivated samples. Thus with two-photon microscopy we probe the bulk minority carrier lifetime of photovoltaic semiconductors. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10× and show that two-photon excitation more accurately measures the bulk lifetime. Finally, we generate multi-dimensional spatial maps of optoelectronic properties in the bulk of these materials using two-photon excitation.
authors.affiliation: The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. esbarnard@lbl.gov
authors.forename: Edward S
authors.initials: ES
authors.lastname: Barnard
authors.forename: Eric T
authors.initials: ET
authors.lastname: Hoke
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Connor
authors.forename: James R
authors.initials: JR
authors.lastname: Groves
authors.forename: Tevye
authors.initials: T
authors.lastname: Kuykendall
authors.forename: Zewu
authors.initials: Z
authors.lastname: Yan
authors.forename: Eric C
authors.initials: EC
authors.lastname: Samulon
authors.forename: Edith D
authors.initials: ED
authors.lastname: Bourret-Courchesne
authors.forename: Shaul
authors.initials: S
authors.lastname: Aloni
authors.forename: P James
authors.initials: PJ
authors.lastname: Schuck
authors.forename: Craig H
authors.initials: CH
authors.lastname: Peters
authors.forename: Brian E
authors.initials: BE
authors.lastname: Hardin